Attribution of changes in winds over the Southern Ocean from 1950 to 2100

Strong near-surface westerly winds drive the Southern Ocean circulation and play a key role in setting regional and global climate. In the latter half of the 20th century, depletion of stratospheric ozone over Antarctica has caused these winds to accelerate and move polewards, particularly in austral summer. However, the future evolution of these winds remains uncertain. We use reanalysis data and the UK Earth System Model (UKESM1), with full atmospheric chemistry, to assess the drivers of winds over the recent past and coming century. We first characterize the wind mean state, distribution, and trends over 1980–2019 in the most commonly used atmospheric reanalyses (ERA5, JRA3Q, and MERRA2) to gain insight into observed wind behaviour in the past. We show that while the representation of the mean wind is similar among reanalyses, MERRA2 shows stronger wind acceleration trends that persist year-round, while JRA3Q and ERA5 show weaker acceleration, primarily in austral summer. Using an observational Southern Annular Mode (SAM) index, we show that the weaker, summer-focused trends of JRA3Q and ERA5 are likely more realistic. UKESM1 represents historical trends in winds accurately compared to ERA5 and is within the range of other CMIP6 models for wind and SAM trends over the historical period. Targeted simulations with UKESM1 show ozone depletion is overwhelmingly responsible for the wind acceleration observed in 1980–2020, primarily in austral summer. The effect of ozone depletion on wind speeds peaks in 1980–2000, when it is roughly double that for the entire 40-year period. Ozone recovery is then associated with a slowdown of winds from 2000 to 2050. Beyond 2050, the ozone effect becomes minimal and winds accelerate primarily due to greenhouse gas induced warming, with this trend more evenly distributed across seasons.